1. Field of the Invention
The present invention relates to an angular rate sensor including a piezoelectric film.
2. Description of the Related Art
There has heretofore been known an angular rate sensor including a piezoelectric film and having an MEMS (Micro Electro Mechanical Systems) structure.
Japanese Patent Application Publication No. Hei 9-178492 (hereinafter Patent Document 1) discloses a piezoelectric vibrator including a piezoelectric vibration element, two driving electrodes and two detecting electrodes. The piezoelectric vibration element is supported at four corners in a vibratingly floating state. In a center part of the piezoelectric vibration element, two drive-side vibration branches are formed so as to extend in a predetermined direction, and two detection-side vibration branches are formed so as to extend in a direction opposite to the drive-side vibration branches. The two driving electrodes are for driving the piezoelectric vibration element and are formed on the drive-side vibration branches. The two detecting electrodes are for detecting an angular rate and are formed on the detection-side vibration branches.
In the piezoelectric vibrator described above, when a voltage is applied to the driving electrodes, the drive-side vibration branches are vibrated. By this vibration, the entire piezoelectric vibration element is vibrated. In this state, when the piezoelectric vibrator undergoes a rotational motion, a Coriolis force acts thereon to generate a voltage in the detection-side vibration branches. By detecting the voltage with the detecting electrodes, an angular rate is detected.
Moreover, there has heretofore been known an angular rate sensor including a piezoelectric film and a cantilever vibrator (see Japanese Patent Application Publication No. 2005-291858 (hereinafter Patent Document 2) and Japanese Patent Application Publication No. 2005-227110 (hereinafter Patent Document 3)).
For example, Patent Document 2 discloses an angular rate sensor having a cantilever vibrator with one end supported on a substrate. The vibrator disclosed in Patent Document 2 includes a reference electrode, a piezoelectric body formed on the reference electrode, a driving electrode formed on the piezoelectric body and a pair of detecting electrodes formed on the piezoelectric body so as to sandwich the driving electrode therebetween.
The vibrator disclosed in Patent Document 2 is vibrated at a resonance frequency in a thickness direction by inputting a driving signal between the reference electrode and the driving electrode. In this state, when the vibrator undergoes a rotational motion, the piezoelectric body is vibrated also in a width direction perpendicular to the thickness direction and is thus deformed. Accordingly, the piezoelectric body outputs a detection signal corresponding to an angular rate due to the vibration in the width direction. This voltage is inputted to control means through the pair of detecting electrodes. Thus, the angular rate is detected.
Here, as means for improving the ability to detect the angular rate by the vibrator as described above, there has been known an approach of setting a ratio of resonance frequencies in the thickness and width directions to be “1” by adjusting the thickness and width of the vibrator.
However, in the piezoelectric vibrator disclosed in Patent Document 1, by vibration of the drive-side vibration branches, the entire piezoelectric vibration element is vibrated and the detection-side vibration branches formed on the side opposite to the drive-side vibration branches are thereby vibrated. Therefore, it is required to vibrate the entire piezoelectric vibration element for vibration of the detection-side vibration branches. The piezoelectric vibrator has a problem of a difficulty in largely vibrating the detection-side vibration branches.
Moreover, as to the vibrator disclosed in Patent Document 2, in order to increase a thickness thereof, a time required for a lamination process is increased. Meanwhile, when a width of the vibrator is reduced, an area of a detecting electrode and a volume of a piezoelectric film are reduced. As a result, an output is lowered. For this reason, there are constraints on improvement in the detection ability of the vibrator by setting the thickness and width of the vibrator to be equal to each other, and thus it is difficult to improve the detection ability in that manner. Furthermore, considering future miniaturization of the vibrator and reduction in the thickness of the film, there is a problem that it is more difficult to improve the detection ability of the vibrator by adjusting the width and thickness of the vibrator.